The present invention relates to a complementary locking system for locking legs to the deck of a jack-up offshore drilling platform, as well as methods for installing one such locking system on the legs of an offshore drilling platform.
Offshore drilling platforms, of the jack-up type, for example oil platforms, generally include legs, substantially three in number, supported on the seabed, together with a fitted deck that can be displaced and adjusted in height along the legs and substantially carrying production equipment and living quarters.
After its construction, the platform assembly is generally floated to the drilling or production site, and the legs are lowered into contact with the seabed, then supported on the legs, the deck is hoisted above sea level up to a height that puts it outside the range of the highest waves.
The deck can thus be displaced along the deck's support legs by means of drive mechanisms housed inside a bearing framework supported by the deck and well-known to specialists as a “jack-house”.
Said drive mechanisms of each support leg each comprise at least two opposing geared motor assemblies driving output pinions that cooperate with opposing racks fitted on the legs.
For this, each leg passes through the deck and the bearing framework of said drive mechanisms and is formed of superimposed sections assembled together by welding. Each section generally consists of vertical chords, three or four in number, interconnected by a lattice of metal girders or by caissons.
Each chord is formed, on the one hand, by a rectangular plate and, on the other, by stiffeners each having the form of a half-shell, each welded onto one of the main faces of said plate.
Each rectangular plate comprises teeth on its lateral faces, which form diametrically opposing racks, intended to cooperate with the output pinions of the drive mechanisms.
Throughout the production run, the platform deck is maintained in its operating position out of the water by the drive mechanisms and more particularly by the pinions of said mechanisms, which engage with the teeth of the support leg racks.
Now the deck, due to its structure and due to the fact that it supports all the equipment, represents a load of several thousand metric tons exerted on the various drive mechanisms.
In addition, the legs, which are solidly anchored onto the seabed, are subject to various marine currents which have the effect of producing shear forces on the leg chords. Said forces obviously have repercussions on the various pinions, in addition to the weight of the platform deck.